1. Field of the Invention
The present invention relates generally to fluid pressure actuated valves and, in particular to a combination by-pass and relief valve which is suited for use in a fluid pressure actuated downhole drilling motor.
2. Description of the Prior Art
Downhole drilling motors of the positive displacement type, embodying a rotor and stator arrangement of the Moineau type illustrated and described in U.S. Pat. No. 1,892,217, are well known. The rotor in prior drilling motors has one lobe operating within a companion two lobe stator made of rubber or corresponding elastomer material, the rotor itself being a solid steel member. The rotor partakes of an eccentric or orbital pass around the axis of the stator, producing an excessive amount of vibration as a result of the orbiting speed of the rotor, combined with its relatively high mass due to its solid construction, resulting in a decreased life of the rotor and of the parts of the motor associated therewith.
The drilling weight of prior motor apparatus is transmitted through a bearing assembly to the motor shaft, this bearing assembly being lubricated by the drilling mud or other fluid pumped down through the string of drill pipe and through the motor itself. Since drilling mud is very often sand laden, the bearings are operating in an abrasive liquid, resulting in their relatively short life, limiting the time that the motor can be used in drilling a bore hole, with consequent requirement for moving the entire motor apparatus from the bore hole and replacement of a substantial number of its parts, or, for that matter, replacement of the entire motor unit. Because of the use of the solid rotor, a dump valve assembly is incorporated in the drilling string above the motor to allow the drilling fluid to fill the drill pipe as the apparatus is run in the bore hole and to drain from the drill pipe while coming out of the hole.
The use of a single lobe rotor results in the rotor, drive shaft and bit connected thereto operating at a relatively high speed, the motor being capable of producing a low maximum torque. Such high speed reduces considerably the drilling life of a drill bit, shortens the life of the bearings, and increases the aforementioned vibration difficulties. With a single lobe rotor, only a limited fluid pressure differential can be used to prevent excessive fluid slippage between the rotor and stator during orbital movement of the rotor around the stator axis with consequent reduction in the horsepower developed by the drilling motor.
U.S. Pat. No. 3,840,080 discloses a downhole drilling motor having a multiple lobe rotor operating within a companion multiple lobe stator. In a Moineau type of apparatus, the stator has one lobe more than the rotor.
With a drilling motor embodying a multiple lobe rotor, the pressure differential that can be used without an undesirable percentage of fluid slippage is far greater than with a single lobe rotor. Accordingly, for a given pressure differential, more drilling weight can be applied to the drilling bit, or conversely, a given drilling weight can be applied to the bit with a less pressure drop across the drilling motor. Since the torque developed for a given pressure is much greater than in the prior drilling motors, and since the capability of greater pressure differential across the motor is present, the combination of these factors results in the capability of the motor to generate a far greater torque than in the prior drilling motors.
By way of example, since the torque generated at any pressure differential in this apparatus is about one and three-fourths times that developed by prior devices, the motor being operable at about twice the pressure differential of the prior devices, the motor is capable of generating at least three and one-half times the torque of the prior devices. Accordingly, while drilling, this apparatus has the capability of operating with about three and one-half times as much drilling weight imposed on the drill bit.
Furthermore, the motor can develop the proper horsepower while operating at much slower speeds than prior fluid motors, permitting roller type drilling bits to be used without increased damage to their parts, so that the drilling bit is capable of drilling greater footages before requiring withdrawal from the bore hole and replacement. The result is a considerable saving in drilling cost per foot of hole and a lesser number of drilling bits being required for drilling a required length of bore hole. Moreover, there is a substantial reduction in the time required for making round trips of the apparatus into and out of the bore hole for the purpose of changing drilling bits.
The vibration of the rotor is considerably reduced by making it hollow, which reduces its mass, thereby contributing to long life of the motor and of the parts associated therewith. The vibration is also reduced by the ability to operate the drilling motor at reduced speeds.
Because of the use of a hollow rotor, with the advantages noted above, a dump valve assembly can be incorporated in the rotor itself, which is closed while drilling fluid is being pumped down through the drilling string and the drilling motor. The valve automatically opens to permit the drilling mud or other fluid to drain from the drill pipe, through the hollow rotor, motor shaft and bit while the apparatus is being removed from a bore hole filled with drilling mud or other fluid, the string of drill pipe automatically filling with the drilling mud or other fluid in the bore hole while the drill pipe and apparatus are being run in the bore hole. The dump valve assembly permits surface checking of the tool without having to insert the tool below the rotary table since the drilling fluid is directed through the passages in the bit, and not of the side of the tool.
The apparatus is provided with a bearing assembly in the drilling motor that is sealed against entry of external fluids and substances, such as the drilling mud. The bearing assembly is filled with oil maintained at a higher pressure than the pressure externally of the bearing assembly, thereby insuring clean oil acting upon the bearings themselves which contributes to the long life of the bearing assembly, enhancing its ability to transmit drilling weight from the drilling string and stator or housing portion secured thereto and to the drill bit, as well as its ability to resist radial or lateral motion of the motor shaft within the stator or housing.